1. Field of the Invention (Technical Field)
The present invention relates generally to the field of maskless direct write deposition of materials, including but not limited to mesoscale electronic structures, using aerodynamic focusing of an aerosolized fluid or particle suspension onto heat-sensitive targets, wherein laser radiation is preferably used to process the deposit to its final state.
2. Background Art
Note that the following discussion refers to a number of publications and references. Discussion of such publications herein is given for more complete background of the scientific principles and is not to be construed as an admission that such publications are prior art for patentability determination purposes.
Various techniques can be used for deposition of electronic materials, however thick film and thin film processing are the two dominant methods used to pattern microelectronic circuits. Thick film and thin film processes for the deposition of electronic structures are well developed, but have limitations due to the high processing temperatures required, or due to the need for expensive masks and vacuum chambers. Thick film processes typically require processing temperatures ranging from approximately 500 to 1000° C. Thin film techniques use processing temperatures ranging from approximately 400 to 3000° C., depending on the type of process and the material deposited. Due to inherent high processing temperatures, thick film and thin film techniques are generally limited to deposition onto ceramic, glass, silicon, and other targets having a damage threshold temperature above approximately 400° C.
Recently, techniques requiring processing temperatures below 200° C. have been developed for deposition of electronic structures on inexpensive plastic targets. One such process for fabrication of transistors on plastic targets is disclosed in U.S. Pat. No. 5,817,550, which uses a pulsed laser processing technique to produce temperatures required for material processing. The laser pulse duration lasts for short periods, limiting the sustained temperature of the target to below 250° C. Another such process for fabricating transistors on plastic targets is disclosed in U.S. Pat. No. 6,642,085, which uses a pulsed laser processing technique similar to that described in U.S. Pat. No. 5,817,550, but is capable of limiting the sustained target temperature to below 120° C.
A thin film process used to form ceramic metallo-organic thin films is disclosed in U.S. Pat. No. 5,064,684. This process casts a liquid metallo-organic ceramic precursor solution to form a layer on a target. The deposit is heated to a low temperature to create an amorphous layer. The process then heats a selected area to a high temperature using localized heating, creating a patterned area of polycrystalline ceramic having electro-optic properties. In U.S. Pat. No. 6,036,889, Kydd uses a mixture of metal powders and metallo-organic decomposition compounds in an organic liquid vehicle to form thick films. The compound is applied to a target using a deposition process such as silk screening, in which bonding is complete at temperatures of less than 450° C.
In U.S. Pat. No. 6,379,745, Kydd, et al. teach a composition having a metal powder or powders of specified characteristics in a Reactive Organic Medium (ROM) that can be deposited to produce patterns of electrical conductors on temperature-sensitive electronic targets. The patterns can be thermally cured in seconds to form pure metal conductors at a temperature low enough to avoid target damage.
In contrast with conventional methods for deposition of electronic materials, the M3D™ process, described in, for example, U.S. patent Publication Nos. 2003/0048314 and 2003/0228124, which are commonly owned with the present application, is a direct printing technique that does not require the use of vacuum chambers, masks, or extensive post-deposition processing. The M3D™ process may be used to deposit a variety of materials with little or no material waste, and has also been used to deposit materials which do not require high temperature processing on low temperature substrates. In order to facilitate this, various low temperature ink systems have been developed. These inks are typically either precursor-based, nanoparticle-based, or they can be combinations of the two. Metal-organic precursor chemistries have a specific advantage in that the precursors can decompose to pure metal at very low temperatures, 150-250° C. range. Because of this the inks can be deposited on many plastics and then heated to decompose to metal. The drawback is that the metal yield of precursor inks is typically low and is in the 1-10% range. The low yield reduces the overall deposition rates.
Metal nanoparticles also have drastically reduced treatment temperatures. Because of their high surface energy, nanoparticles will melt at temperatures hundreds of degrees lower than micron-sized particles. Nanoparticle inks in particular have been shown to sinter in the 150-250° C. range. The metal yield of nanoparticle inks can be in the 10-50% range, which leads to highly efficient deposition. For example, the M3D™ process has been used to deposit and laser process silver (at 150° C.) on an FR4 substrate, which has a damage threshold of less than 200° C., with no damage to the substrate.
While considerable progress has been made in low-temperature ink development, the sintering temperatures are still significantly higher than the softening temperature of many common plastics. For example PMMA softens at around 100° C. and most nanoparticle and precursor inks will not become conductive or ductile at this temperature. In addition, it is difficult if not impossible to avoid thermal damage to a target if the processing temperature of the deposited material exceeds the damage threshold of the target. The types of damage possible when polymer targets are subjected to excessive heat are warping, vaporization of volatile components, oxidation, decomposition, burning, softening, and melting. Glasses may undergo crystallization and melting, and metals may undergo oxidation, recrystallization, grain growth, reversed hardening, and melting, when subjected to excessive heat. Ceramics may also undergo thermal damage in the form of unfavorable phase changes that may lead to cracking or loss of material or electrical properties, vaporization of volatile components, and oxidation (for ceramics that are not oxides). For example, densification on low temperature substrates has only been achievable for materials that can be densified at temperatures below the damage threshold of the substrate.
Thus there is a need for an apparatus and method to deposit and process materials at a nearby or higher temperature than the damage threshold of the target or substrate.